In both terrestrial and submarine environments there are situations in which the transmission of imaging information through the field of view is limited rendering it difficult to characterize surfaces with conventional imaging system components such as, for example, a CCD-based imaging device and a divergent illumination source. One common limiting factor is the presence of a large number of suspended particles in the field of view. Not only does this result in significant back scattering of light, but it also contributes to transmission loss of imaging detail. Typically, when the predominant component of energy received by the imaging device is attributable to scattered light, the signal-to-noise ratio is too low to provide useful information.
Several designs and configurations have emerged for underwater imaging applications at varied ranges. Conventional camera systems having an adjacent broad spectrum light source are useful for imaging surfaces at distances of one to two attenuation lengths. An attenuation length is the distance light must travel to be reduced to 1/e of its original intensity. The attenuation length is typically 20 to 30 m in clear water. It has been found that at distances of about three attenuation lengths, acceptable imaging can be provided by spatially separating the light source from the camera, e.g., using a flood light to illuminate the target region.
At imaging distances greater than three attenuation lengths, laser-based systems are more effective. These extended range imagers are generally of two classes: the synchronous laser line scanner (LLS) and the range gated scanner. See Jaffe,]; S. et al. “Underwater Optical Imaging: Status and Prospects”, Oceanography, Vol. 14, No. 3 pp. 66-76 (2001). See, also, U.S. Pat. Nos. 4,707,128 and 5,418,608 each of which is incorporated herein by reference. These types of imaging systems can provide acceptable real-time image data in the range of 3 to 7 attenuation lengths. Such imagers have been under continued development for use in Autonomous Underwater Vehicles (AUVs) and Remotely Operated Underwater Vehicles (ROVs) to provide surface information needed for navigation as well as for characterizing the sea floor for varied activities including military missions and construction of oil and gas infrastructures.
Synchronous LLS systems provide scanning capability with a continuous wave (CW) laser source. Based on results of controlled experimentation and analytical modeling, synchronous scanners have been found capable of operation at maximum distances of about 5 to 6 attenuation lengths. Further improvement in imaging range would benefit undersea operations by allowing increased vehicle speed and maneuverability and improved image resolution at greater distances from target regions. By way of example, in the exploration of unknown or dynamic environments, rapid topographical seabed variations can occur at rates greater than the vertical axis performance of the AUV. It is therefore necessary to distance the vehicle at a sufficient range above the seabed to avoid potentially catastrophic collisions. Optimal underwater optical scanner designs must consider this AUV trajectory.
The ability to more rapidly produce higher resolution images of targets and survey sites from greater distances will enable a more extensive and diverse set of applications for underwater vehicles. Depending on the size and complexity of surfaces in the target region, optical sensing may be the only effective means for characterizing features.
It has also been shown by both simulation and experimentation that the class of range-gated imagers i.e., those imagers utilizing a pulsed laser source, may be capable of adequate underwater performance for imaging target regions at distances up to seven attenuation lengths. These systems minimize introduction of energy due to scattered light with gating electronics. Although these imagers ultimately become power (or photon) limited due to the exponential decay rate of light traveling through the water, they can be more compact than CW LLS systems because a spatial offset between the source and receiver is not required to reject scattered light.
Summarily, both classes of extended range underwater imagers ultimately are limited in range by the cumulative effects of forward scattering events and divergence of the illumination, particularly as the reflected signal travels from the target region to the imaging system. Scattering causes losses in contrast, resolution and signal to noise ratio (SNR). These losses are particularly problematic at and near the range limit of operation.
Relatively small depth of field (DOF) has also been a disadvantage in prior LLS system designs. This is particularly problematic when imaging in a dynamic undersea environment in which there is significant variation in optical transmission properties or in sea bed surface features or in which there is significant variation in platform altitude or attitude. With a small DOF each of these factors can lead to unacceptable degradation in image quality or complete signal loss. The DOF is a function of the source-receiver separation distance, the optical path length to and from the target, beam divergence and the acceptance angle of the receiver. The receiving aperture of the LLS system may be widened to improve DOF. Alternately, a fine adjustment of the optical focus may be slaved in accord with an on-board altimeter.
Range-gated imagers have also had inherent disadvantages in addition to limitations in imaging distance. For example, variations in distance between the system and a target surface result in a change in the required delay time of the gating function used to selectively acquire photons returning from the target.
Based on the foregoing it is apparent that both classes of extended range imagers have performance limitations restricting usefulness in a variety of potential applications including, for example, smoke-filled environments, fog, adverse weather conditions and underwater imaging. In addition, the size, weight and power requirements are also extremely important when designing an imager for portable or mobile deployment in any of the afore-described environments.
The optical resolution achievable with a LLS system is dependent on the laser beam diameter at the reflecting surface in the target region, and is also dependent upon the precision with which the receiver can resolve intensity information from the return signal as a function of the scan angle. Minimizing the instantaneous field of view (IFOV), e.g., by minimizing the spot size at the target, reduces the scattering volume, which reduction can improve the signal-to-noise ratio. That is, the imaging range of the system can be improved by reducing the size of the scattering volume. Reducing the IFOV reduces the target area per pixel, commonly measured in cm2 per pixel and, theoretically, improves image resolution. This is particularly desirable when imaging target surfaces having a high spatial frequency, as the combined effects of forward scattering and blurring, due to the limited DOF, further limit the achievable resolution.
The '821 patent describes a synchronized laser beam scanning system in which the scanning architecture is built around a single six faceted polygonal scan mirror. The system provides a very narrow instantaneous field of view (IFOV) at the receiver channel which is optically coincident with the outgoing laser pulse throughout the entire scan angle for a fixed stand-off distance. Using two symmetrical steering mirror assemblies, one for the outgoing beam and one for the returning signal, optical synchronization can be maintained as the stand-off distance is adjusted. The symmetry of the source and receiver channels about the center axis of the polygon also significantly reduces the necessary size of the detector photocathode area required to complete a full scan through a wide angle. Polygonal mirror systems are widely used in other laser scanning systems. See, for example, U.S. Pat. No. 6,723,975. However, provision of mechanically rotatable polygonal mirrors in scanners poses a significant addition to the system size and cost and may affect reliability. Efforts to build small, more compact laser line scanners of this type are subject to limitations because of the mechanical nature of the rotating mirror systems. Another intrinsic limitation of the raster scanning based techniques such as the LLS system is that in order to maintain the image resolution with increased platform speed, higher laser repetition rate will be required. This in turn affects the system cost and complexity (i.e., noise mitigation of wider bandwidth electronics).
U.S. Pat. No. 7,609,875, referred to herein as the '875 patent, also incorporated herein by reference, discloses a Micro-Electro-Mechanical Systems (MEMS) based laser scanning system having a MEMS mirror which can oscillate in two independent directions. A high speed modulated or pulsed laser beam is transmitted through a fiber collimator and is then radiated toward the MEMS mirror which reflects the beam through a fixed optical path, consisting of a beam splitter, a lens and a static mirror, to the target. Light reflected from the target traverses the same fixed optical path in a reverse direction before entering the receiver. The system as disclosed in the '875 patent does not include any measures to mitigate signal impairment due to volume backscattering or other light scattering phenomena.